The present invention relates to seals in a gas turbine for supplementing the chordal hinge seals between turbine nozzles and a turbine nozzle support ring and particularly relates to supplementary seals for substantially minimizing or eliminating leakage losses past the chordal hinge seals.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles typically include an annular array or assemblage of cast nozzle segments each containing one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer band portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array thereof between annular inner and outer bands. A nozzle retaining ring coupled to the outer band of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular nozzle support ring, preferably split at a horizontal midline, is engaged by the inner band and supports the first-stage nozzles against axial movement.
In an exemplary arrangement, eighteen cast segments are provided with two vanes per segment. The annular array of segments are sealed one to the other along adjoining circumferential edges by side seals. The side seals seal between a high pressure region radially inwardly of the inner band, i.e., compressor discharge air at high pressure, and the hot gases of combustion in the hot gas flow path which are at a lower pressure.
Chordal hinge seals are used to seal between the inner band of the first-stage nozzles and an axially facing surface of the nozzle support ring. Each chordal hinge seal includes an axial projection which extends linearly along a chord line of the inner band portion of each nozzle segment. Particularly, the chordal hinge seal extends along an inner rail of each segment and which rail extends radially inwardly of the inner band portion. The chordal hinge seal projection lies in sealing engagement with the axially opposite facing sealing surface of the nozzle support ring.
During operation and/or repair of the first-stage nozzle, it has been found that warpage can leave gaps between the chordal hinge seals and the sealing surface of the nozzle support ring. These gaps enable leakage past the chordal hinge seals from the high pressure area radially within the annular inner band into the hot gas flow path. That is, the chordal hinge seals are inadequate to prevent leakage flow as the chordal hinge seal projections lose contact with the sealing surface of the nozzle support ring. Consequently, there is a need for a supplemental seal at the interface of the first-stage nozzles and nozzle support ring to minimize or eliminate the leakage flow past the chordal hinge seals.
In accordance with a preferred embodiment of the present invention, there is provided a supplemental seal between the first-stage nozzles and the nozzle support ring which eliminates or minimizes leakage past the chordal hinge seals. The supplemental seal includes a flexible, preferably sheet metal seal secured between the inner rail of each nozzle segment and the sealing surface of the nozzle support ring on the high pressure side of the chordal hinge seal. Particularly, the supplemental seal has a first margin which is sealingly engaged in a groove formed along an inner chordal surface of the inner rail and a second margin which has an arcuate configuration projecting axially toward and for sealing engagement against the sealing surface of the nozzle support ring. The first margin of the supplemental seal received in the groove is bent or folded to bear in sealing engagement against a surface, preferably the base of the groove to preclude leakage past the seal at the groove. To retain the margin of the supplemental sheet metal seal in the groove, an elongated strip is secured to the flexible seal linearly along its first margin. The groove has a flange and the strip includes an opposing, overlapping flange which retains the flexible seal against radial dislocation from the groove. Preferably, a second elongated strip is provided along the opposite side of the first margin from the first strip to locate the edge of the strip centrally within the groove of the inner rail.
The second or opposite margin of the supplemental seal includes an edge which bears against the sealing surface of the nozzle support ring. The second margin is arcuate about the axis of the rotor. The supplemental seal thus seals directly between the high pressure region and regions radially outwardly of the supplemental seal, including the chordal hinge seal. Thus, leakage flow past the chordal hinge seal is minimized or eliminated as a result of the sealing margins of the supplemental seal against the inner rail and the nozzle support ring. Preferably, the supplemental seal has a linear first margin such that the first margin can be disposed along a chord line within the groove along the inner surface of the inner rail. The opposite edges of the supplemental seal may overlap with one another to form inter-segment seals.
In a preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a turbine nozzle segment having at least one stator vane and including an inner band carrying an inner rail, the inner rail having a second surface in axial opposition to the first surface, one of the first and second surfaces including an axially extending projection therealong for engagement with another of the first and second surfaces to form a first seal therebetween sealing between high and low pressure regions on opposite sides of the seal and a flexible supplemental seal extending between the inner rail and the first surface at a location radially inwardly of the first seal and between at least opposite end edges of the segment, a first margin of the flexible seal being secured to the inner rail and a second margin of the flexible seal sealingly engageable with the first surface.
In a further preferred embodiment according to the present invention, there is provided a gas turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a plurality of turbine nozzle segments each having at least one stator vane and an inner rail, the inner rails of the segments forming a generally annular second surface in axial opposition to the first surface, each of the segments including an axially extending projection along the second surface thereof for engagement with the first surface to form a first seal therebetween sealing between high and low pressure regions on opposite sides of the seal, a plurality of flexible supplemental seal segments extending between the inner rails and the first surface at locations radially inwardly of the first seal, each flexible seal segment having a first margin secured to the inner rails of at least two adjoining segments and spanning at least the joint therebetween, a second margin of each flexible seal segment sealingly engageable with the first surface in response to leakage flow past the first seal.